198 WILLIAM BRODBECK HERMS 



to twenty seconds before, were sufficient to keep the larva mov- 

 ing in the general direction taken at first (fig. 7a). Less fre- 

 quently the larva withdrew after a few random movements and 

 then took a course along the border of the field either to the right 

 or to the left (figs. 6 right hand tracing and 7b), or in a very few 

 cases turned directly back on its own course. 



Apparently'- the "starting off" was brought about by means of 

 an intensity too high in proportion to the non-directive light. 

 Therefore in all later experiments a directive light of about 0.5 

 CM. was employed. Under such conditions the larvae very sel- 

 dom went into the field of light : but, once in the field, they passed 

 through in the manner described, again with no constant relation to 

 intensity. It was then decided not to turn off the directive light, 

 but keep it on the larva continuously. The result of these experi- 

 ments is shown in fig. 8, which represents the usual reaction either 

 to right or left. The larva on arriving at the edge of the field of 

 higher intensity withdrew and then moved somewhat transversely 

 to the directive rays which however, soon caused it to take up its 

 usual longitudinal orientation, as a result of which if once more 

 entered the more intense field, but again withdrew. This behavior 

 continued until the region of higher intensity was passed where- 

 upon the course finally followed the rays of the directive light. 



Over 150 recorded experiments, as illustrated by the figures 

 were made for fields varying from 10 cm. to 2 cm. in length. An 

 equal number of unrecorded experiments {i.e. unrecorded by larval 

 traces) were made in which the light passing through the plate 

 glass stage was reflected out of the light grader by means of the 

 mirror always in place. There is clearly no constant relation 

 between the movements of the animals and the graded field. 



It was further found that the larvae could be forced entirely 

 through the graded field from the dark (0) end to the brightly 

 illuminated end (70.69 CM.) when an intensity of 50 CM. or 

 over was used to direct them. This was not possible when the 

 directive light had an intensity of only 0.5 CM. Under this in- 

 tensity the larvae would, however, pass farther into the darker end 

 of a field grading at the rate of 7 CM. per centimetre, than into 

 one grading at the rate of 35 CM. per centimetre. Though 



